Instrument Description

Last update : 2016-09-22

The SPHERE ETCs (IRDIS, IFS and ZIMPOL), the three “exposure time calculators” were updated during period 97 to better predict peak fluxes or the DIT (for a given setup and conditions set) at which the star will saturate (without coronagraphs), and are now considered reliable.

The SPHERE ETCs and the previous ETC excel sheets are both valid calculators now (see ETC for IRDIS/IFS: DIT_calculator_irdis-ifs_v5.xlsx & ZIMPOL: DIT_calculator_zimpol_v2.xlsx, updated 2015-07-01). The excel sheets are still made available since they are more convenient in some cases. Authors: Julien Girard, Julien Milli, Arthur Vigan & Zahed Wahhaj (+ help from the consortium): sphere@eso.org

These ETC tables were originally made to address significant discrepancies between ETC saturation predictions and actual SPHERE data. If you want to use the ETC excel tables, please refer to the information below to estimate roughly the telescope time you need for your project (Phase 1) or refine your choices at the time of making your OBs (Phase 2).

Reference mag/DIT Tables & ZIMPOL “beta ETC”:

For Phase 2 & VM runs/operations, we elaborated several tables for the most generic setups of each instrument. This tables, based on real data acquired during the various commissioning, technical and SV runs.

FLUX measurements (non-CORO): non-coronagraphic photometric reference images should always be taken in the linearity range of the detector. For IRDI(F)S that’s below 20,000 ADU and for ZIMPOL that’s below 50,000 ADU. The final accuracy of the relative photometry measurement between a star and its companion is dominated by the measurement error on the faint companion as long as the “F(lux” measurement is performed with sufficient SNR (typ. at least a few hundreds or a few thousands ADU on the peak). Therefore it is better to be conservative and aim for rather ~25-50% of the non-linearity limit (typ. 5,000 to 10,000 ADU for IRDIS and IFS) rather than aiming for ~50% and risking to pass the limit or saturate in case the conditions are very good.

COROnographic measurements: we are talking about the flux that leaks out of the coronagraphic mask, the “batman ears”. It is quite difficult to estimate their maximum flux as they heavily depend on the centering accuracy and the AO performance in a non-linear way. For example, if the AO performances degrade, a lower strehl ratio will induce a lower peak flux (lower level of the FLUX measurement) but it might induce and increase in leak flux as the wider PSF (halo contribution) is leaking out more around the mask. Empirically we could reach an understanding for a limited number of setups. For IRDIS DBI we consider the highest flux across all spectral channels. Here we aim at 1/3 to 2/3 of the saturation range, not as conservatively as for the F(lux) measurements. The reason is that we think it is better to slightly degrade the inner-working angle (IWA, < 0.2” only reachable in certain cases) than underperforming in most of the control zone (0.2-0.9”).

This table provides estimations of the recommended DIT to reach about 20 000 ADU on the zimpol detector (saturation at 64 000 ADU) given the filter, ND, coronagraph and star magnitude. Only the worksheet “Estimator” is useful to the user.

The user must provide his/her input in the blue cells, by choosing the star magnitude in R, and the instrument setup (string values to be chosen among the lists described in columns I, J and K). The output, provided in the green cell, gives the ideal DIT in this configuration. The user must select the appropriate neutral density (abbreviated ND) to make sure this DIT is above the minimum DIT corresponding to the instrument setup.

The tables provide an estimation of the recommended DIT and neutral density to be used for your observations as a function of the target magnitude. They are provided for the IRDIFS, IRDIFS_EXT, IRDIS DBI and CI modes. The values are presented separately for the FLUX observations (off-axis reference PSF) and the OBJECT observations (coronagraphic images). Be careful to choose your DIT/ND combination so that the number of expected counts is always below 20000 counts (linearity limit). For the FLUX observations, it is recommended to aim for 10000-15000 counts to be sure to avoid any saturation that would the off-axis PSF unusable for subsequent analysis. For OBJECT observations, a slight saturation at the edge of the coronagraph can be expected (possible loss of inner-working angle but no under-performance in contrast accross the control radius of the DM).

The average value of the neutral density filters attenuation factor is also provided for convenience if you want to adopt different DIT/ND configuration.